This invention generally relates to a compressor inlet valve, and more particularly to a compressor inlet valve for electronically controlling inlet gas flow and preventing backflow through the compressor inlet.
In order to control the throughput or capacity of a compressor, a compressor typically includes an inlet valve which regulates the compressor capacity. One type of inlet valve is commonly referred to as an unloader valve because the valve is used to load and unload the compressor. The compressor is loaded when the inlet valve is open permitting fluid, such as air, to flow through the compressor inlet. The compressor is unloaded when the valve is closed thereby "choking" or blocking the flow of fluid through the compressor inlet.
Unloader valves may be opened and closed pneumatically. Pneumatically controlled unloader valves require a regulation air system for operation. Although the pneumatically controlled unloader valves have operated with varying degrees of success, there are problems associated with such valves. When the compressor is operated in temperatures that are below freezing, the regulation air system may freeze and render the inlet valve inoperable. Additionally, the regulation air system requires regular maintenance in order to ensure that the air system can effectively actuate the unloader valve during compressor operation. This regularly conducted maintenance can be time consuming and may render the compressor inoperable when it is being performed.
Unloader valves may also be opened and closed hydraulically. Hydraulic unloader valves frequently leak hydraulic fluid and require replacement of parts, such as diaphragms, for example.
A problem associated with compressors, especially oil-flooded screw compressors, is backflow through the compressor inlet. Such backflow is comprised of a combination of a gas, such as air, and oil. Backflow occurs when the compressor is stopped while the compressor system is pressurized. It is undesirable to permit backflow to be released into the environment because of the loss of oil from the system and associated contamination of the environment. One conventional way of preventing backflow is by inserting check valves in the air service and oil injection lines. Conventional check valves are spring actuated to permit unidirectional flow of compressed gas or oil, away from the compressor. In this way, backflow is prevented by the check valves. Although current check valves are effective in preventing backflow, it would be more desirable to prevent backflow without introducing additional discrete valves into the system. The addition of the discrete check valves increases the cost and complexity of the compressor. In hydraulically and pneumatically operated unloader valves, the backflow may be used to close the unloader inlet. However, the tendency to freeze, problems with leaking oil and hydraulic fluid and required maintenance make hydraulically and pneumatically operated unloaders undesirable.
Electronically operated inlet valves typically include a stepper motor that is connected to a disc or piston that is movable by the motor. A pressure sensor measures compressor discharge pressure, generates a signal in response to the measured pressure and communicates the signal to a controller. In response to the signal generated by the sensor, the controller calculates the distance that the disc or piston needs to be moved to obtain the desired discharge pressure and rotates the stepper motor in short, discrete angular movements to thereby move the disc or piston the calculated distance. Typically, the disc or piston when fully closed, does not seal the inlet well enough to prevent backflow of oil. To date, compressors with electrically actuated inlet valves do not seal against backflow and require that a discrete check valve be inserted in a compressed air service line, typically located downstream from the compressor discharge port along with another check valve, known in the art as an oil stop valve, located in an oil injection line. These valves increase the cost and complexity of the compressor.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.